I generally have an idea of what an item is worth to me. I could care less what the price guides say an item is worth; ultimately it’s how much am I willing to spend to own the item.
If the seller has obviously priced his items to sell and I believe the price to be fair; I’ll pay the sellers price.
If I believe the seller has some wiggle room; I’ll often just ask “what would you be willing to do on this?” If I find that number to be acceptable, I’ll buy. If not, I’ll generally say something like “nah; that’s more than I was thinking.” In most cases; the seller will then ask what I think is fair and the negotiation begins.
If I believe an item to be egregiously overpriced, I simply move on without expressing or showing any interest. Odds are I’ll either find it at a lower price at some point or, I really didn’t need it anyhow.
Curt